The disclosure relates generally to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion drilling assemblies. Still more particularly, the disclosure relates to percussion drilling assemblies with a locking mechanism to prevent disengagement of a driver sub from a case.
In percussion or hammer drilling operations, a drill bit mounted to the lower end of a drill string simultaneously rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A percussion drilling assembly typically includes a piston-cylinder assembly coupled to the hammer bit. Impact force is generated by the piston-cylinder assembly and transferred to the hammer bit. During drilling operations, a pressurized or compressed fluid, e.g., compressed air, flows down the drill string to the percussion drilling assembly. A choke is provided to regulate the flow of the compressed fluid to the piston-cylinder assembly and the hammer bit. A portion of the compressed fluid flows through a series of ports and passages to the piston-cylinder assembly, thereby actuating the reciprocal motion of the piston, and then is exhausted through a series of passages in the hammer bit body to the bit face. The remaining fraction of the compressed fluid flows through the choke and into the series of passages in the hammer bit body to the bit face. The compressed fluid exiting the bit face serves to flush cuttings away from the bit face to the surface through the annulus between the drill string and the borehole sidewall.
A typical percussion drilling assembly 10 is illustrated in FIG. 1. Percussion drilling assembly 10 is connected to the lower end of a drill string 15 and includes a top sub 20, a driver sub 40, a tubular case 30 axially disposed between top sub 20 and driver sub 40, and a hammer bit 60 slidingly received by driver sub 40. Top sub 20 is threadingly coupled between drill string 15 and case 30. A piston 35 is slidably disposed in case 30 above hammer bit 60. A fluid conduit 50 extends between top sub 20 and the upper end of piston 35, while a guide sleeve 25, positioned in case 30 axially above driver sub 40, slidingly receives the lower end of piston 35.
Compressed fluid is delivered from drill string 15 through top sub 20 and fluid conduit 50 to upper and lower piston-cylinder chambers to actuate piston 35. As is known in the art, percussion drilling assemblies may alternatively utilize an air distributor assembly, from which air is directed into the upper and lower piston-cylinder chambers 39, 38, respectively. As compressed fluid is alternatingly delivered to upper and lower piston-cylinder chambers 39, 38, piston 35 is actuated to cyclically impact hammer bit 60. Guide sleeve 25 maintains fluid communication between piston 35 and bit 60 as piston 35 moves relative to hammer bit 60.
The lower end of case 30 is threadingly coupled to the upper end of driver sub 40. Hammer bit 60 slideably engages driver sub 40. A series of generally axial mating splines on bit 60 and driver sub 40 allow bit 60 to move axially relative to driver sub 40 while simultaneously allowing driver sub 40 to rotate bit 60 with drill string 15 and case 30. A bit retaining ring 45 is disposed about the upper end of hammer bit 60 above driver sub 40, and a retainer sleeve 55 is coupled to driver sub 40 and extends along the outer periphery of hammer bit 60. Bit retaining ring 45 prevents hammer bit 60 from completely disengaging assembly 10. The retainer sleeve 55 generally provides a secondary catch mechanism that allows the lower enlarged head of hammer bit 60 to be extracted from the wellbore in the event of a breakage of the enlarged bit head.
During drilling operations, percussion drilling assembly 10 may be in one of three positions: on-bottom, fully closed; off-bottom, fully open; and on-bottom, partially closed (or off-bottom, partially open). In the on-bottom, fully closed position, bit 60 engages the formation and is installed in driver sub 40 with a shoulder 65 of bit 60 abutting the lower end of driver sub 40. This is the optimal position for drilling when sufficient weight is applied to bit 60. In the off-bottom, fully open position, bit 60 does not engage the formation and is fully extended relative to driver sub 40, but prevented from disengaging assembly 10 by bit retaining rings 45. In this position, percussion drilling assembly 10 ceases to impact the formation. In the on-bottom, partially closed (or off-bottom, partially open) position, drill bit 60 is partially, but not fully, extended such that shoulder 65 of bit 60 does not engage the lower end of driver sub 40. This is the non-optimal drilling position that occurs when insufficient weight is applied to bit 60 and may result in loosening of threaded connections within percussion drilling assembly 10.
Motions of the percussion drilling assembly during operation and vibrations resulting from repeated impact of piston 35 with bit 60 and of bit 60 with the formation can cause threaded connections, such as the threaded connection between case 30 and driver sub 40, to become loose and unthread. Other situations that promote unthreading may be improper connection make-up at the surface, inconsistent or inadequate weight-on-bit during drilling operations, and improper operation of the hammer due to downhole conditions. In the event that driver sub 40 disengages case 30, drilling operations cannot continue. Instead, case 30 must be pulled from the borehole, and a costly fishing operation ensues to retrieve driver sub 40 and any other component which has also disengaged from assembly 10, such as drill bit 60 and bit retaining ring 45. If the disengaged components cannot be retrieved, it may be necessary to redirect or sidetrack the borehole before drilling may continue.
Accordingly, there is a need for locking mechanisms to prevent unthreading and disengagement of a driver sub from a case within a percussion drilling assembly.